Many efforts have recently been made to minimize the sizes and prices, and at the same time maximize the function, of high frequency devices including a high frequency wireless device, satellite broadcast receiver, and a wireless LAN system. Accordingly, demands are high for smaller, thinner, and cheaper electronics components for high frequency circuits incorporated in those devices. In addition, a protrusion, such as an antenna, on a portable device is not a desirable feature for aesthetic and portability reasons.
In a move to respond to these demands, dielectric ceramic small block antennas have been introduced into market.
Nevertheless, the dielectric ceramic small block antenna has an antenna pattern formed directly on a dielectric, providing a relatively narrow sensitive band width. Also, if the user carries the antenna in his hand and holds it close to his ear, the antenna is affected by the human body in terms of dielectric constant. The result is a disturbed electromagnetic field and a mismatched impedance, which in turn cause trouble in communications. To prevent such a problem from happening, the antenna is connected to a high frequency circuit via an impedance matching device.
Furthermore, since the high frequency circuit generally contains a distribution constant circuit that handles mismatching of the impedance, an unexpected electromagnetic coupling may possibly occur between a signal line and a closely placed GND line. The distribution constant circuit in some cases has an unexpected coupling capacity between wiring lines due to irregularity in dimensions of the wiring pattern and other reasons. These are causes for the disruption of the impedance from being matched in the high frequency circuit.
The irregularity in dimension precision of the wiring pattern (circuit pattern) needs to be solved by trimming and improvement in the precision of the print pattern of the distribution constant circuit. Therefore, strict control is needed on the length of lead wires and the amount of solder when electronics components are mounted. As briefly illustrated here, the distribution constant circuit for impedance matching has a problem that high precision is demanded in its manufacturing process.
The high frequency circuit therefore very often includes an impedance matching device between circuit blocks as well as in the connection with the antenna, since the inclusion reduces reflected waves and better transmits signals.
A conventional impedance matching device installed in high frequency circuits is typically large in size because of a capacitor and a coil incorporated therein for adjustment.
Moreover, so as to impart the characteristics required by the circuit module to the impedance matching device, consideration needs to be paid to the applicable temperature range and the temperature characteristics of various components in the design of the circuit. Skilled engineers and their experiences were essential to such a design process. Should the design fail to produce desirable characteristics, complex adjustment such as trimming is inevitable.
Therefore, conventional impedance matching devices have a problem of complex design and manufacturing processes and accompanying high costs.